Components exposed to elevated temperatures and mechanical stresses, such as aircraft engines which typically employ nickel, iron or cobalt based superalloys, require protective coatings from corrosion, oxidation and from the high operating temperatures to achieve reliable operation for extended periods of time. More specifically, component surfaces having metallic heat rejection coatings, such as platinum, gold or rhodium which may be sandwiched between a pair of stabilizing layers, such as tantalum, that are exposed to radiative flames exhibit both measurable temperature decrease and increased service life compared to uncoated component surfaces. These heat rejection coatings achieve this temperature decrease by effectively reflecting the radiative energy away from the component surface rather than have this energy absorbed. Accordingly, it is highly desirable to apply these heat rejection coatings to similarly exposed surfaces. However, this is not possible for certain metal alloy parts, such as turbine nozzles, which may be regularly exposed to temperatures exceeding about 788° C. (1450° F.). At this temperature range, the heat rejection coating interdiffuses with the underlying substrate. In essence, a portion of the heat rejection coating material migrates into the component substrate material while elements of the substrate migrate outwardly into the coating material. This interdiffusion causes the reflective heat rejection surface to become a radiation absorber, losing or at least substantially losing its ability to reflect radiative energy, resulting in a reduction of its ability to decrease component surface temperature, thereby decreasing the service life of the component. Therefore, a means to prevent the interdiffusion between component surfaces and the heat rejection coatings is highly desired.
One method to prevent high temperature interdiffusion is the provision of a barrier coating applied between the component surface and the heat rejection surface. A variety of these barrier coatings are known in the art and include paint-on dielectric oxides, chemical vapor deposited oxides and baked-on rare earth oxides. However, none of these barrier coating constructions may be utilized in this application because they either are inefficient in preventing high temperature diffusion or lose their adhesive properties at higher temperatures.
Alternately, it has been shown that nozzles may be covered with a thick macroscopic coating of a ceramic thermal barrier coating, (“TBC”), one type of which is known as “smooth coat,” commonly employing a TBC composition referred as “(AJ11).” U.S. Pat. Nos. 5,624,721 and 5,824,423 are directed to methods which employ aluminum bond coat layers for securing TBC coatings. While heat rejection coatings have been shown to remain intact when applied over these TBC coatings, the thick TBC coatings are expensive to manufacture and apply and are extremely heavy, effectively limiting their application to aerospace components.
Thus there is a need in the art for an inexpensive, lightweight means to prevent interdiffusion between component surfaces and heat rejection coatings.